Aggression is central to many aspects of social behavior. One form of aggression found in many animals, including man, is territoriality. By looking at territorial behavior in a group of animals, different from man, anthropomorphic bias will be avoided and inherent characteristics of territoriality can be determined. The proposed research is a pilot study designed to provide a new and innovative approach to the study of territoriality in birds. By combining behavioral theory with the experimental manipulations of resources and animals in the field, I expect to unwind the paradigms of territoriality in the White-throated Sparrow, Zonotrichia albicollis. Through experimental manipulations, I will determine if White-throated Sparrows are territorial for the purpose of obtaining an adequate food supply. Utilizing a bivariate home range model, I will estimate the territory size of individual birds, for each stage of the breeding cycle. Perturbations will be made during the nestling stage when White-throated Sparrows generally forage by scratching at the ground to uncover insects. In the territories of two groups of birds I will place polyethylene sheets on a portion (30% or 50%) of the preferred foraging areas. In this manner I will lower the amount of food available to these animals. I will add food to the territories of one group, and from the nests of one group of birds I will remove two nestlings, thus lowering the energy demand of these birds. A final group of animals will serve as controls for the experiment. Differences in the (1) changes in territory size and (2) individual fitness (i.e., number of successfully fledged young) among the experimental and control animals will indicate if their procurement of an adequate food supply is one function of territorial behavior in the White-throated Sparrow.